When to Use "That" as a Conjunction
Part of our jobs as bloggers and copywriters is to make our writing as tight as possible. One of the words that often gets the strike by writers and editors is the word "that," when used as a conjunction.
Sometimes the edit is warranted. For example, "I knew you had gone to the meeting" is just as clear as "I knew that you had gone to the meeting." The sentence is just tighter if you omit "that."
However, sometimes omitting "that" can make a sentence incredibly confusing. For example: "Police believe a well-known serial killer is behind the murders." In this sentence, I'd first read: "Police believe a well-known serial killer," which makes me think that maybe they're taking in his testimony. Then the rest of the sentence throws me for a loop and I have to read the sentence over again for it to make sense.
The role of an editor, in addition to making sure that writing is grammatically correct, is also to make things as easy on the reader as possible. So in this case, I'd change the sentence to say: "Police believe that a well-known serial killer is behind the murders." Adding "that" removes all ambiguity from the first part of the sentence.
Here is the ruling from the AP Stylebook on using "that":
Use the conjunction “that” to introduce a dependent clause if the sentence sounds or looks awkward without it. There are no hard-and-fast rules, but in general:
• That may usually be omitted when a dependent clause immediately follows a form of the verb to say: “The president said he had signed the bill.”
• That should be used when a time element intervenes between the verb and the dependent clause: “The president said Monday that he had signed the bill.”
• That usually is necessary after some verbs, including: advocate, assert, contend, declare, estimate, make clear, point out, propose and state.
• That is required before subordinate clauses beginning with conjunctions such as after, although, because, before, in addition to, until and while: “Haldeman said that after he learned of Nixon’s intention to resign, he sought pardons for all connected with Watergate.”
When in doubt, include "that." Omission can hurt. Inclusion never does.